This invention concerns a catheter with a cylindrical stent of a permeable mesh of stiff intersecting fibers. When the stent is used it expands on its own due to its radial elasticity from a tense condition with a small circumference into a relaxed state supporting the vascular wall with a large circumference that is uniform over the length. A tubular outer catheter shaft holds the stent under tension at the distal end in such a way that the stent can be released from the catheter in order to be used. A displaceable internal catheter inside the tubular outer catheter shaft supports the stent axially at the proximal end thereof, whereby the outer catheter shaft is retracted with respect to the internal catheter in order to release the stent.
Such catheters are known, for example, from U.S. Pat. No. 4,655,771. They are used to position vascular supports, vascular endoprostheses or so-called stents in vessels in the human body. Recently, a special field of use for these stents has developed in conjunction with increased use of percutaneous transluminal coronary angioplasty (PTCA). In this technique, a catheter is inserted into a blood vessel through a puncture in the skin and is advanced through the blood vessel up to an arteriosclerotic occlusion in coronary vessels, for example. An inflatable balloon is attached to the end of the catheter. This balloon is inflated and the occlusion in the blood vessel is enlarged by this pressure. Then the balloon can be deflated again and the catheter removed from the body. In most cases, the blood vessel then remains open for continued blood flow.
One main complication of this technique, however, is that it causes detachment of parts of the intima, the innermost layer of the vascular wall, from the vascular wall so they then interfere with flow in the vessel to various extents. In the worst case the fragments of vascular wall released in this way can act like a valve to completely obstruct the flow passage. At certain locations, e.g., in the coronary arteries, such an occurrence can lead to a critical situation necessitating an emergency bypass operation which entails a high risk for the patient. However, even at other treatment sites and with a less unfavorable course of the complication, the desired result of the treatment is in any case hindered by this complication.
In the case of such complications, the stents known for some time, e.g., from U.S. Pat. No. 4,655,771, have been used at the treated site in the vessel to hold the vessel open from the inside. To do so, a catheter is inserted into the blood vessel through the same puncture already used for the balloon catheter. The stent is inside this catheter on the distal end relative to the user. The stent is cylindrical and consists of a network of stiff intersecting fibers. It is the self-expanding type, i.e., it is inserted into the catheter under tension and then relaxes on its own without assistance. Other types of stents must be converted to their expanded form by means of an interior balloon. At the treatment site the stent is released from the catheter by retracting the outside catheter and then detaching the stent from the catheter. A displaceable internal catheter inside the catheter serves as a support for the stent as long as the outside catheter is retracted. The stent remains in the vessel after being released and thus provides permanent support for the vessel. However, the catheter is retracted as usual and the puncture site in the vessel is sealed.
Such stents usually fulfill their purpose by pressing the intima, the innermost detached layer of the vessel, back against the vascular wall and thus they keep the vessel open for the flow of blood. However, problems occur since these stents can cause blood clots. This danger must be counteracted with high doses of anticoagulants which are also potentially dangerous. After a few weeks, the stent is then overgrown by the vascular intima, the endothelium, and the danger of blood clots is thus largely eliminated. But now a new problem has occurred. It has been found that the tissue cells whose growth is stimulated by the introduction of the stent cannot stop growing in some cases, so this results in a new partial or complete occlusion of the vessel.
At the same time, it is known that detached intima can become attached to the vascular wall again within a relatively short period of time and can heal there. In some cases renewed brief inflation of the balloon at the end of the aforementioned balloon catheter is sufficient to accomplish this. While the balloon is under pressure, the flow of blood in the respective vessel is interrupted, so this method cannot be used at all treatment sites. In addition, healing times are prolonged due to the need for anticoagulants during treatment.
In order to eliminate the problems described above, catheters have already been described where the stent is used to support the vessel wall only temporarily and then can be removed from the vessel again.
An example of this is given in European Patent 0 321 912 A1, which concerns a stent consisting of mesh tubing of woven wires that can be stretched longitudinally and inserted into the vessel. Then at the treatment site the two ends of the mesh tubing are advanced toward each other so the mesh bulges out between the two ends to form a hollow shape which presses against the inside wall of the vessel and thus supports it. The mesh of which this stent is made is thus not self-expanding but instead is stretched in its relaxed state. In this relaxed but stretched state with a small circumference, the stent is inserted into the vessel and removed from the vessel again after use. The pressure of the hollow form on the vascular wall varies according to how much the ends of the mesh tubing are advanced toward each other.
One disadvantage of this design, however, is that the individual wires can bend when the ends of the mesh tubing are pushed together too strongly and then the wires cannot yield in the vessel. Complications occur when removing a catheter with bent wires from a vessel because the actuating elements of the mesh can transmit compressive forces only to a limited extent in order to return the mesh to the elongated form with a small circumference. Another disadvantage is that the mesh is gathered at both ends so the blood flow must pass through the mesh twice when it is in position in the blood vessel, i.e., once at the proximal end of the stent and the second time at the distal end of the stent. Another disadvantage is that the actuating forces holding the mesh tubing open during the duration of the treatment must be maintained over a relatively great distance from the outside. This can result in transmission errors when, for example, the catheter is advanced between the point of puncture into the skin and the point of treatment.
Another example of a stent that can be removed from the body is disclosed in World Patent WO 91/07928 where the stent consists of a single wire coiled into a helical shape. The wire is stretched and accommodated in a thin catheter tube from which it is also advanced forward. As soon as the wire comes out of the thin catheter tube at the distal end, it assumes its spiral shape or helical shape again because of the stresses imprinted on it. The individual coils of the helical wire press radially outward and thus support the vascular wall. To remove this stent the wire is again retracted into the catheter. The wire then returns to its elongated form. This stent is thus the self-expanding type.
With the self-expanding type there is no danger of excessive operating forces having a negative effect on the stent or even rendering it useless. The flow conditions for blood are favorable with this type of stent, as can be imagined, because the helix is open at the distal end from the catheter and thus the blood must flow through the windings of the helix only once.
Use of just one wire, however, has the disadvantage that the wires of the stent must be very close together in order to effectively support a large area of vascular wall. In a helical form with just one wire, however, the windings do not hold together in such a way that would assure uniformly close spacing of the windings. This can result in gaps in the support provided for the vascular wall. Another unpleasant disadvantage is that the helical stent does not remain stationary on insertion and removal from the vessel. On insertion and removal of the stent, the relaxed portion of the helical spring must rotate with respect to the wire in the catheter in order to compensate for the difference in the condition of the wire. The freely rotating end of the wire can cause damage to the vascular wall when positioning the stent. However, the main disadvantage is that the rotation of the helical stent as it is being positioned can cause it to slip under a detached flap of vascular wall and thus prevent the stent from fulfilling its function. This stent is therefore definitely not as effective as the known stent that remains permanently in the vessel. In addition, the high frictional forces of the wire when it is under tension in the thin catheter also cause problems. Therefore, a great resistance must be overcome in order to expel the stent from the catheter, and this resistance may be further increased by the pressure of the wire against the catheter wall when it is being expelled.
Another example of a stent that can be removed from the vessel is disclosed in European Patent 0 423 916 A1. This is a slidable lattice grate in the form of a segment of tubular sheathing made of stainless steel wire. This stent is also the self-expanding type and is inserted into the vessel by retracting an outside catheter with respect to an inside catheter exactly like the stent according to U.S. Pat. No. 4,655,771. On the proximal edge of the slidable lattice grate that forms a segment of tubular sheathing, a thread is provided on the edge of the tubing. With this thread the tubing can be gathered at the proximal end. In order to accomplish this, both ends of the thread extend outside the body where they are secured loosely as long as the stent remains in the body. When the stent is to be removed, a new catheter is advanced as far as the stent by means of these two threads and the proximal end of the stent is tightened by pulling on these threads accordingly. Then a second larger catheter is advanced over the first. The stent is then gathered with these threads until it fits inside the larger catheter and can be inserted into it. Next, the two catheters together with the stent are removed.
One advantage of this arrangement is the good flow achieved when the stent is in place in the vessel because the segment of tubing is open at both ends. A disadvantage is the great effort required for this arrangement. The procedure is very tedious due to the handling of the threads and the need to insert at least one new catheter which also must be advanced into the proper position by means of the threads. The instrumental expense is also very high because at least one additional catheter must also be provided to accommodate the stent again. The catheter used for insertion of the stent is too small for this purpose. When the stent is inserted into coronary arteries, problems can also be expected on insertion of the stent into the larger catheter because the coronary arteries are in constant motion. Problems therefore come about due to the fact that the stent, which is gathered together at the proximal end, is not always positioned exactly in the middle with respect to the larger catheter, nor is it automatically centered with respect to the larger catheter. Therefore, the stent remains stuck at the edge of the larger catheter.